The inventions relate to the field of linearizing amplifier predistortion. More particularly, the present inventions relate to high power amplifier predistortion at baseband based on improved Saleh modeling and linear-log modeling.
High power amplifiers (HPA) suffer from the effects of amplitude modulation to amplitude modulation distortion (AM-AM), also referred to herein as amplitude distortion, and amplitude modulation to phase modulation distortion (AM-PM) also referred to herein as phase distortion, during conversions caused by the HPA amplifiers, such as solid-state power amplifiers (SSPA) or travelling wave tube amplifiers (TWTA). Amplitude distortion and phase distortion are major concerns to communication systems engineers. These distortions can cause intermodulation (IM) components and spectral regrowth, which are undesirable to system designs. The intermodulation components and the spectral regrowth can cause adjacent channel interference to other communication channels. Furthermore, these distortion effects also cause loss in power transmission. In addition, the effects of AM-AM and AM-PM distortions can also cause the signal distortion that can degrade the bit error rate (BER) performance of a communication channel.
When a baseband signal of a constant envelope QPSK signal is filtered into a filtered baseband input signal, the resultant modulated QPSK signal no longer has the constant-envelope property. If the filtered QPSK signal is then passed through a HPA operating at saturation, the spectral regrowth is present at the output of the HPA. Similarly, when two constant envelope signals are combined and then passed through a power amplifier operating at saturation, the properties related to spectral regrowth and IM products become uncertain. It has been shown that the spectral regrowth and power loss in the IM products can cause severe BER degradation. In order to reduce spectral regrowth and BER degradation associated with HPAs operating at saturation, an HPA linearizer is needed.
The amplitude and phase modulation distortions are minimized using linearization method. The linearization method minimizes the mean square error function between input and output. The linearization method requires modeling the characteristics of the amplitude distortion and phase distortion of the HPA. A Saleh model and a linear-log model have been used to provide the linearization method. The Saleh model has been applied to measured data from HPAs that characterize the distortion caused by the HPAs. The measured data provides a performance curve indicating nonlinear distortion. The measured data is curved fitted by selecting Saleh coefficients inserted into the Saleh model to then obtain a forward Saleh model that approximately fits the measured performance curve to a Saleh model curve. The forward Saleh model with the Saleh coefficients models the distortion of the HPA. The forward Saleh model is a math equation that describes the amplitude and phase modulation distortions of the HPA. The amount of desired predistortion is then determined to inversely match the amount of distortion for canceling out the distortion of the HPA. To determine the amount of predistortion, an inverse Saleh model is generated that mathematically describes the amount of predistortion necessary to inversely match the amount of distortion of the HPA. A mean square error function is defined by the difference between the forward Saleh model and inverse Saleh model and is used to compute the amount of predistortion adjustment needed. In order to compute the amount of predistortion needed, the inverse Saleh model is expanded into a power series. The power series is needed for reducing the mean square error function to improve the accuracy of the computed predistortion characteristics. The predistortion characteristics are then applied to an input signal to reduce amplitude and phase modulation distortions of an output signal relative to the input signal. However, the Saleh model method is not flexible enough to match a wide variety of HPA distortion characteristics. The Saleh model method requires high processing capabilities for power series expansion for minimizing the error of computed distortion by generating a power series for each particular HPA. As such, the Saleh model method does not offer a general solution for a wide class of HPAs. For different amplifiers having different distortion characteristics, the computations for predistortion using the Saleh model require substantial processing power.
The linear-log model also uses measurement data that characterizes the nonlinearity distortion of the HPA, and then stores predistorter characteristics in look-up tables that map the amount of predistortion to the input amplitude to reduce amplitude and phase modulation distortions. However, the linear-log model can not control clipping effects caused by operation of the HPA at or near saturation where desirable maximum input power is provided. To address this problem, the input is limited so as to avoid the saturation output operating point. As input signal amplitudes vary from signal to signal, and vary over time, and as HPA performance drifts over time, the linear-log model method requires adjusting the input signal amplitudes so as to avoid saturation clipping so as to maintain desired bit error rates. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide linearization of the output of a high power amplifier using a predistorter.
Another object of the invention is to provide linearization of the output of a high power amplifier using a predistorter operating at baseband.
Another object of the invention is to provide linearization of the output of a high power amplifier using computed parameters that adjust the desired predistortion.
Another object of the invention is to provide linearization of the output of a high power amplifier using computed backoff parameters that changes the slope of a linearization curve of a predistorted HPA so that input signal can be processed at a normalized 0 dB.
Another object of the invention is to provide a linearization of means using a double set of Saleh extended coefficients to compute in closed-form the desired amount of predistortion.
Yet another object of the invention is to provide linearization of the output of a high power amplifier using an adjustable slope linearization for reducing unwanted amplitude modulation and phase modulation caused by amplitude variance of an input to the amplifier.
Another object of the invention is to reduce the occupied bandwidth of signal produced by an amplifier using baseband predistortion linearization.
Still a further object of the invention is to reduce bit signal to noise ratio degradation due to phase distortion of signal produced by an amplifier using baseband predistortion linearization.
Yet another object of the invention is to reduce spectral regrowth due to amplitude distortion and phase distortion by an amplifier using baseband predistortion linearization.
The present invention is directed to predistorters for the linearization of a high power amplifier (HPA) to mitigate the amplitude modulation to amplitude modulation distortion (AM-AM), that is, amplitude distortion, and amplitude modulation to phase modulation distortion (AM-PM), that is, phase distortion, and the distortion effects in communication systems, particularly digital communication systems. The linearization methods of the invention are advantageous to systems requiring improved HPA linearization performance. In a first aspect of the invention, an extended Saleh method is used, and in a second aspect of the invention, a modified linear-log model is used, both to reduce amplitude and phase distortions. Both methods can be implemented using an architecture having real to complex conversion prior to predistortion and complex to real conversion after predistortion. The predistortion is accomplished through complex processing operating at baseband as a complex baseband linearizer providing reduced spectral regrowth and simplicity in design.
The extended Saleh method invention offers a general solution including additional polynomial terms rendering an expression that is applicable to a wide class of HPAs, using a double set of extended Saleh coefficients without expanding the inverse Saleh model into the power series for any particular HPA, but enables the use of coefficients for particular HPAs entered into a general solution. The extended Saleh method offers improved flexibility to accommodate a wide class of HPA while operating at baseband. The desired predistortion can be computed in closed-form reducing processing power for the computation of the desired amount of predistortion. Whereas, the modified linear-log model reduces clipping when operating the HPA at saturation when the input is provided at a normalized 0 dB value. The modified linear-log method specifies the amount of acceptable loss of output power based on a normalized 0 dB input signal so that the input signal need not be controlled to avoid saturation. In the event of HPA drifts or varying input power level, voltage parameters can be computed and used to modify the slope of the linearization curve so as to avoid saturation and clipping. In this manner, the linear-log model method determines the linearization curve that minimizes clipping effects while maximizing the normalized output power level. The extended Saleh method and the modified linear-log method offer improved flexibility to accommodate operation at baseband with reduced system complexity of the transmitters. The extended Saleh and modified linear-log methods operating at baseband offer reduced spectral regrowth out of the communication bandwidth thereby reducing the occupied bandwidth for improved co-channel isolation. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.